1. Field of the Invention
The present invention relates to a semiconductor device comprising a ferro-electric capacitor or ferro-electric memory composed of highly dielectric materials.
2. Description of the Related Art
Year after year the integration of a semiconductor device has been sophisticated and miniaturization of the circuit used therein has been promoted. Caused by such demand it is required that capacitors maintain their volume while the effective area be smaller and therefore, it is required that films of the capacitors be thinner or highly dielectrics materials be used for the capacitors. With respect to the thickness of the films, the films have reached their lowest possible thickness and electric field strength, which are next to the dielectric breakdown, and it is impossible to make the film thinner. Accordingly, use of dielectrics is indispensible.
Highly dielectric materials mean materials having a higher relative dielectric constant than that of SiO2 and Si3N4 which have been used for the prior capacitor. Such materials are generally dielectric oxides and when development of such materials was started, monometallic oxide such as Ta3O5 was examined. In recent years, perovskite dielectrics oxide such as SrTiO3, BaxSr1-xTiO3 (BST), PbSrxTi1-xO3 (PZT), Pb1-yLayZrxTi1-xO3 (PLZT), and SrBi2Ta2O9 have been studied and if such dielectrics are realized, a capacity of 500 times or more that of the prior capacitor, can be ensured.
In connection with formation of ferro-electric memory, particularly, use of highly dielectric materials such as PZT can contribute to producing ferro-electric nonvolatile memory (FeRAM) that cannot delete information though external voltage is cut off, and so it has been attracting attention. Ferro-electric materials have spontaneous polarization and an electric field can reverse its orientation. Since ferro-electric materials generally have an ABO3 type perovskite constitution (here, A represents one or more elements chosen from Ba, Sr, Pb, La, or divalent metal and B represents one or more elements chosen from Ti, Zr, or tetravalent metal), electric field having the reverse orientation needs to be impressed in order to transfer atoms at B sites to the other stable sites. Therefore, ferro-electric materials show a hysteresis characteristic and even when the electric field is 0, they maintain residual polarization and so are suitable and can be used as a memory. However, when the above mentioned dielectric oxide was employed for forming capacitors and memory, the following problems are produced.
Forming films of the dielectric oxide in an oxidizing atmosphere causes the first problem. Dielectric oxide is formed into films by a sol-gel method or a CVD (chemical vapor deposition) method. With respect to the sol-gel method, gels of metallic compound composing dielectric oxide are applied on the substrate by spinning and the substrate is dried, and then a high temperature, heat treatment is performed for the purpose of crystallizing dielectric oxide, and the heat treatment is carried out in an oxidizing atmosphere to prevent oxygen from being lost. The sputtering method is conducted in plasma containing oxygen and so, this method employs a so-called reactive sputtering style. The CVD method is performed by utilizing such energy as heat, plasma and light, and all these processes are carried out in an oxidation atmosphere to prevent oxygen from being lost.
Since an electrode film of ferro-electric capacitors is composed of the inoxidizable platinum family including Pt, or metals, that are conductive even when they are oxidized, such as Ir, Ru, or Os, forming a film of dielectric oxide in an oxidation atmosphere does not cause any problems. What causes problems is that oxygen infiltrates the electrode film and diffuses while it is being formed, and whereby a contact plug made of polycrystalline Si connected to the electrode film and a barrier layer made of Tin are oxidized. When the contact plug made of polycrystal Si and a barrier layer are oxidized, problems have been brought about in that resistance at the electrode is increased and adhesion is lowered, and accordingly, a ferro-electric capacitor thus produced cannot satisfy required performance and its yielding percentage is poor. For instance, K. Kusida-Abdelghafar et al. reported that when a barrier layer made of TiN was laminated on the Si substrate and thereon a Pt crystal film having a columnar structure was formed as a lower electrode film, while a PZT thin film was being formed, oxygen diffused through the grain boundary of Pt crystals, having a columnar structure and constituting the lower electrode film, over the surface of a TiN film and TiO2 was formed in the midst of Pt in Mater. Res., 1998, Vol.13, p.3265.
The cause of oxygen infiltrating the lower electrode film easily is that the electrode film manufactured by a well-known prior method is composed of crystal grain layers having a columnar structure. Since the crystal grain layers in a columnar structure have large crystal grain diameters and crystal grain boundaries exit along the direction of electric conduction, conductivity is favorable, but oxygen permeability is high as well and so, the oxygen barrier performance is low.
As a means to enhance the oxygen barrier performance of the electrode film Matsui et al. disclosed a method for forming an electrode film, using Pt crystal having a granular structure, which had been difficult in the past and made a report that the oxygen barrier performance was improved compared with that of an electrode film having a columnar structure in the proceedings of the lecture meeting of the 44th Applied Physics Society Association, 1997, Vol. 2, p.437. However, since the granular structure is composed of micro-crystal grains, compared with the columnar structure, its oxygen barrier performance was improved, but resistivity was also higher and resulted in conduction failure. Further, crystallinity of the dielectric oxide thin film is affected by that of the touching electrode film and accordingly, when Pt in a granular structure, which is inferior in terms of crystallinity to a columnar structure, was employed as an electrode film, crystallinity of dielectric oxide is also worsened and resulted in lower specific inductive capacity and decreased residual polarization.
The second problem is conduction failure and adhesion failure occurring at the connected surface between respective films, caused by inter diffusion of constituent substances including oxygen occurred through the electrode films between the dielectric oxide thin film and contact plugs or barrier film. This problem causes not only difficulty while the films of dielectric oxide are being formed but also economical difficulty. For instance, the life of dielectric capacitors is shortened and reliability is lowered. More specifically, after a ferro-electric capacity is manufactured, inter diffusion of constituent substances including oxygen gradually occurs through the electrode films and results in conduction failure and peeling at the connected surfaces between respective layers. When the above-mentioned Pt crystals having a columnar structure are used as the electrode films, sufficient conductivity of currents is secured, but substance transfer through grain boundaries of the columnar structure occurs relatively easily. On the other hand, when the electrode films are composed of crystals having a granular structure, barrier performance is high, but conductivity of current is low and the films were not good for practical use.
The third problem is that, when ferro-electric memory is formed of dielectric oxide, rewritability is insufficient. Dielectric oxide has so-called fatigue characteristics, namely a repetition of reverse polarization results in a decrease in residual polarization. The chief causes of the fatigue characteristics are diffusion of the metals constituting the electrode films into the dielectric oxide thin films, leakage currents through crystal grain boundaries in the dielectric oxide thin film, and dispersion of oxygen atoms (oxygen holes) within the lattices of the dielectric oxide thin film. These causes can be eliminated if the crystallinity of the dielectric oxide thin film can be improved. More specifically, if crystallinity of the dielectric oxide thin film is high enough, no defective or amorphous part exists and perfection of crystals is high. Consequently, crystal grain boundaries are small and the substance diffusion into the dielectric oxide thin film, leakage current, generation of oxygen holes are inhibited. To put a ferro-electric memory into practical use, it is essential that a decrease in residual polarization caused by repeated polarization is inhibited and rewritability is improved.
The object of the present invention is to provide a semiconductor device, wherein electrode films have a crystal grain multi-layer structure containing a columnar structure and a granular structure. Another object of the present invention is to provide a semiconductor device which is 1) equipped with a ferro electric capacitor comprising electric films with both favorable oxygen barrier performance and good current conductivity, or 2) said ferro-electric capacitor maintains good performance for a long time, or 3) equipped with a ferro-electric memory having high rewritability by a method easier and more multi-purpose than investigating a new metallic material for the electrode films.
Further, another object of the present invention is to inhibit an increase in resistance and deterioration of adhesion at the electrodes by improving anti-oxidation characteristics of a barrier film.
A semiconductor device according to the present invention comprises a thin-film capacitor having a dielectric oxide thin film and a pair of electrode films sandwiching said dielectric oxide thin film therebetween. At least one of said pair of electrode films is composed of a crystal grain laminated structure containing a columnar-structure crystal grain layer and a granular structure crystal grain layer.
A semiconductor device according to the another aspect of the present invention comprises a thin film capacitor having a dielectric oxide thin film and a pair of electrode films sandwiching said dielectric oxide thin film therebetween, a contact plug connected to at least one of said electrode films, and a barrier layer formed between said electrode film and contact plug. Said barrier layer is a granular structure crystal grain layer made of tantalum nitride containing 10 atm % or more and 50 atm % or less of nitrogen.
Here, crystal grain layers mean layers made of aggregated crystal grains having the same crystal grain shape. Crystal grain shapes mean the shapes of respective crystals (crystal grains) constituting polycrystal if metallic materials are formed of polycrystals, and there are two kinds of crystal grain shapes of columnar or massive. A granular structure means a thin film structure having columnar crystals growing on the substrate, as stipulated by JIS No. H0211. FIG. 3 illustrates a schematic diagram showing a columnar structure, however the present invention is not limited hereto. A granular structure means a thin film structure having massive crystals growing on the substrate and FIG. 4 illustrates a schematic diagram showing a granular structure, however, the present invention is not limited hereto. Actual conditions of crystal grain layers and crystal grain shapes can be perceived by observation with a scanning electron microscope (SEM).
Also, a semiconductor device according to still another aspect of the present invention comprises a MOS transistor formed on the semiconductor substrate, an interlayer insulating film formed on said mos transistor, a contact plug disposed in said interlayer insulating film and connected to a diffusion layer contained in said MOS transistor, a lower electrode film formed on said contact plug, a dielectric oxide thin film formed on said lower electrode film, and an upper electrode film formed on said dielectric oxide thin film, wherein said lower electrode film is composed of a crystal grain laminated structure containing a columnar structure crystal grain layer and a granular structure crystal grain layer.
In addition to the above, a semiconductor device according to still another aspect of the present invention comprises a MOS transistor formed on the semiconductor substrate, an interlayer insulating film formed on said MOS transistor, a contact plug disposed in said interlayer insulating film and connected to a diffusion layer contained in said MOS transistor, a barrier layer formed on said contact plug, a lower electrode film formed on said barrier layer, a dielectric oxide thin film formed on said lower electrode film, and an upper electrode film formed on said dielectric oxide thin film, wherein said barrier layer is a granular structure crystal grain layer made of tantalum nitride containing 10 atm % or more and 50 atm % or less of nitrogen.
Here, a contact plug is illustrated with a MOS transistor formed on the semiconductor substrate, an interlayer insulating film formed on this transistor, and a contact plug made of polycrystalline Si filling an aperture portion that is disposed in this interlayer insulating film so as to reach a diffusion layer of said MOS transistor. Polycrystalline Si is often used for contact plugs, but tungsten (W), tungsten silicide (Wsix), and titanium silicide (TiSix) are also employed.
In the present invention, a barrier layer may be formed between said lower electrode film and contact plug. The barrier layer is used to prevent substance from diffusing mutually among such films as the semiconductor substrate, electrode films, and dielectric oxide thin films, and to improve adhesion between the film and TiN and TiSi2 are preferable for the barrier layer. The barrier layer can consist of either a single-layer or multi-layers.
Moreover, in the present invention, it is preferable for a crystal grain layer which is contained in said crystal grain laminated structure and touching said dielectric oxide thin film to have a granular structure, however, the present invention is not limited hereto. As examples of the crystal grain laminated structure of the present invention, when starting the description from the film touching the dielectric oxide thin film, columnar/massive, such examples of massive/columnar, columnar/massive/columnar, massive/columnar/massive, and a large number crystal grain layers are provided.
A method of manufacturing a semiconductor device according to the present invention comprises the steps of forming columnar structure crystal grain layers by a sputtering method or a CVD (Chemical Vapor Deposition) method and forming massive crystal grain layers by the sputtering method or the CVD method, in forming electrode films having crystal grain laminated structure containing columnar structure crystal grain layers and granular structure crystal grain layers.
The forming crystal grain layers by the sputtering method in the present invention is explained concretely as follows. A target is placed, facing the semiconductor substrate and in an Ar gas atmosphere high frequency waves are generated between the semiconductor substrate and the target to discharge electricity and to accumulate a substance constituting a thin film on the semiconductor substrate. After that by heating and annealing it the accumulated substances is crystallized and forms an electrode film having prescribed shapes and diameters of the crystal grains. There is a case of the reactive sputtering method wherein, nitrogen gas, which constitutes a thin film as an ingredient, is mixed with Ar gas to make the accumulating substance absorb gas constituents. This procedure is applied to produce an electrode film made of tantalum nitride. Various conditions including the kinds of target employed, pressure of Ar gas, frequency of high-frequency waves, temperatures of discharging electricity, and film forming temperatures are optimized by the kind of shapes and diameters of the crystal grains to be formed.
The forming crystal grain layers by the CVD method in the present invention is explained concretely as follows. Mixture of the substance constituting a thin film and highly volatile substance, particularly organic metallic compounds composed of trimethyl, tri-isobutyl or dimethylhalide, and metals are adsorbed on the semiconductor substrate and then such volatile substances are removed. Since removal of volatile substances and crystallization is conducted under heating conditions and take advantage of reactions occurring on the surface of the semiconductor substrate, atoms adsorbed on the semiconductor substrate move on the surface and a film excellent in coating an uneven surface can be formed. Various conditions including the kind of the organic metallic compounds employed and a temperature of forming films are optimized by the kind of shapes and diameters of the crystal grains to be formed.
The forming an electrode film composed of either a single columnar structure crystal grain layer or a single granular structure crystal grain layer is already publicly known, however, forming a granular structure crystal grain layer was relatively more difficult than forming a columnar structure crystal grain layer, due to limits on the most suitable conditions. It has been very difficult to form an electrode film having a multiple crystal grain layer structure since the most suitable conditions were more limited. However, as explained above, by bringing the manufacturing conditions under strict control, it became possible to manufacture such an electrode film. In the present invention, by adopting crystal grain structure to an electrode film that contains crystal grain layers both in the columnar structure and granular structure, the following effects can solve the problems explained above.
Firstly, a problem, wherein since a dielectric oxide thin film is formed in an oxidization atmosphere, diffusion of oxygen occurs while the electrode film made of columnar structure crystal grain layers are being formed, whereby oxidization of a contact plug, connected to an electrode film, or a TiN barrier layer is caused, is solved. More specifically, if crystal grain layers having crystal grain shapes in the granular structure exists in the electrode film formed in a crystal grain laminated structure, oxygen does not pass through the electrode film, for the diffusion speed of oxygen infiltrating the boundary of each crystal grain is extremely slow. However, since micro-crystal grains constitute the granular structure, although the oxygen barrier performance is excellent, resistance is also high and results in conduction failure. On the other hand, since in a crystal grain layer formed in the columnar structure, the diameters of crystal grains are large and crystal grain fields exist along the direction of electric conduction, resistance is low and conductivity is good. Therefore, by laminating crystal grain layers constituted by these different crystal grain shapes and forming an electrode film, both of the oxygen barrier performance and conductivity, good for practical use, can be realized.
The performance and productivity needed for electrode films determine what shape grains should a crystal grain layer have and how many layers should be laminated and in what order they should be done. By controlling the crystal grain laminated structure of the electrode films, the electrode films that have both the oxygen barrier performance and conductivity, good for practical use, can be provided by a method suitable for multiple purposes, which is easier than developing new constituents of the electrode films. As a result, while the films of dielectric oxide are formed, the contact plugs or barrier layers cannot be oxidized and resistance at the electrodes does not increase. Adhesion to the electrode films remains to be good, as well. Secondly, an aging problem, wherein after the dielectric oxide thin film is formed, interdiffusion of constituent substances including oxygen is occurred through the electrode film between the dielectric oxide thin film and contact plug or barrier film, and resulting in conduction failure or peeling on the jointed surface between respective films, is solved. More specifically, if crystal grain layers having the crystal grain shapes in the granular structure exist in the electrode film formed in a crystal grain laminated structure, diffusion of the substance including oxygen can be prevented and no substance including oxygen can pass through the electrode film. On the other hand, while the crystal grain layers having a crystal grain shapes in the columnar structure have low substrate barrier performance, they have good conductivity. It takes considerable time and cost if development of a new electrode film is started from search of new materials constituting the electrode film, for the purpose of developing the new electrode film that can have both of the excellent substance barrier performance and conductivity, which are good for practical use. Compared with the above, production of the electrode film formed in a crystal grain laminated structure containing the crystal grain layers both in the columnar structure and the granular structure can realize both the barrier performance and conductivity even thought its constituents remain unchanged as they are now. As a result, oxidization of the contact plug or barrier layer while the dielectric oxide thin film is being formed can be prevented and moreover, such aging problems as the insufficient rewritability of dielectric memory can be solved, as well.
Thirdly, fatigue characteristics of the dielectrics oxide material can be improved. The main causes of the fatigue characteristics include diffusion of the metals constituting the electrode film into the dielectric oxide thin film, leakage currents by way of crystal grain fields in the dielectric oxide thin film, dispersion of oxygen atoms (oxygen holes) in the lattice of the dielectric oxide thin film. These causes can be solved if crystallinity of the dielectric oxide thin film is improved, and the following explains how it should be realized. That is, the crystallinity of the dielectric oxide thin film is affected by the crystallinity of the electrode film it touches. Accordingly, by controlling the crystal grain shapes and diameters of the crystal grains in the electrode film that touches the dielectric oxide thin film and crystallizing such crystal grain layers, crystallinity of the dielectric oxide thin film can be improved. Generally speaking, since the crystal grain layers in the columnar structure have better crystallinity than those in the granular structure, the crystal grain layer touching the dielectric oxide thin film can be made to have a columnar structure when necessary. If the crystal grain layer next to the above layer is made to have the granular structure, the substance barrier performance can be obtained. By making the electrode film have a crystal grain multi-layer structure, composed of a plurality of crystal grain layers, and controlling such crystal grain multi-layer structure, all the performance required of the electrode film such as crystallinity, conductivity and the substance barrier performance can be satisfied.
Fourthly, anti-oxidation characteristics of the barrier layer can be improved. That is, when the barrier layer is formed of a granular structure crystal grain layer made of tantalum nitride containing 10 atm % or more, preferably 15 atm % or more, more preferably 20 atm % or more and 50 atm % or less of nitrogen, preferably 45 atm % or less, more preferably 36 atm % or less of nitrogen, its anti-oxidation characteristics can be improved.
Further, in the case where the electrode film is formed in the above-mentioned barrier layer, c-axis orientation of the crystal grains constituting the electrode film improves and the number of routes through which oxygen infiltrates decreases. Therefore, the oxygen barrier performance of the electrode film further improves and oxidization of the barrier layer can further be inhibited. Also, since the dielectrics oxide to be formed on the electrode film whose c-axis orientation is improved has further improved crystallinity, the fatigue characteristics of the obtained FeRAM can further be improved and the obtained DRAM can have larger capacity.
As explained above, by making the electrode film have a crystal grain multi-layer structure, the semiconductor device of the present invention can realize the good substance barrier performance, good current conductivity and high crystallinity at such electrode film by a multi-purpose manufacturing method, which is easier than investigating new constituents for the electrode film. As a result, such dielectric capacitor can work well without causing any deterioration of its performance even after it is used for a long time and a semiconductor device can have dielectric memory with good rewriting performance.
In addition, by making the barrier layer to be a granular structure crystal grain layer made of tantalum nitride containing 10 atm % or more and 50 atm % or less nitrogen, anti-oxidation characteristics of the barrier layer can be improved, and whereby resistance of the barrier layer cannot be increased, whereas its peeling can be prevented.